The present invention relates to semiconductor lasers, and in particular, to a semiconductor laser to be used for excitation in an optical fiber amplifier.
FIG. 25A is a sectional view of a ridge type semiconductor laser mentioned in JP Hei.11-233883A1. In a semiconductor laser, which is generally indicated by the reference numeral 170, an n-type GaAs substrate 101 is used as a substrate. The GaAs substrate 101 is laminated with an n-type AlGaAs cladding layer 102, an undoped AlGaAs second guide layer 103, an undoped GaAs first guide layer 104 and an undoped InGaAs active layer 105, the layers being stacked in this order. Further, an undoped GaAs first guide layer 106, an undoped AlGaAs second guide layer 107 and a p-type AlGaAs cladding layer 108 are laminated in this order in positions symmetrical about the active layer 105. The p-type AlGaAs cladding layer 108 has a ridge structure for causing current constriction. A p-type GaAs contact layer 110 is formed on a ridge portion 109. Further, an insulating layer 111 is formed so that the p-type GaAs contact layer 110 is partially exposed, and a p-side electrode 112 that is electrically connected to the exposed portion of the p-type GaAs contact layer 110 is formed on the insulating layer 111. On the other hand, an n-side electrode 113 is formed on the opposite surface of the GaAs substrate 101.
The semiconductor laser 170 shown in FIG. 25A is formed so that a refractive index (n1c) of the n-type AlGaAs cladding layer 102 is made greater than a refractive index (nuc) of the p-type AlGaAs cladding layer 108 (FIG. 25B). Therefore, a light intensity distribution of laser light generated in the active layer 105 becomes a distribution in which a peak position is shifted toward the GaAs substrate 101. Consequently, a far field pattern (FFP) in the x-axis direction of the light intensity distribution becomes narrowed, allowing the laser light aspect ratio (xcex8v (xcex8 in the x-axis direction)/xcex8H (xcex8 in the y-axis direction), xcex8: full angle at half maximum) to be reduced.
Moreover, the light intensity distribution toward the ridge portion 109 is reduced, and the discontinuous refractive index distribution of the ridge portion 109 exerts less influence. Therefore, a kink that occurs due to a change in a lateral mode, i.e., a discontinuous point of an optical output caused by a change from fundamental mode light emission into first order mode light emission is raised to allow a stabilized emission light intensity to be obtained.
However, the semiconductor laser 170 shown in FIG. 25A has had the problem that a variation efficiency (dP/dI) of the optical output with respect to a variation in current is great with regard to the kink level attributed to a change in a longitudinal mode, i.e., a current-to-optical-output characteristic of the semiconductor laser, and thereby, a stabilized light intensity cannot be obtained. In particular, if the semiconductor laser is used as a semiconductor laser for excitation in an optical fiber amplifier, then a fluctuation in light intensity is amplified. Therefore, the stabilization of light intensity has been a serious problem. There have been the further problems of an increase in threshold current, a reduction in optical output efficiency and so on occurring when the semiconductor laser comes to have an elevated temperature as a consequence of light emission.
FIG. 26 shows a current-to-optical-output (P-I) characteristic and a current-to-efficiency (dP/dI-I) characteristic of the semiconductor laser 170 shown in FIG. 25A. The horizontal axis represents the current (I), and the vertical axis represents the optical output (P) and the efficiency (dP/dI). The line (a) indicates a relation between the current (I) and the optical output (P), and the line (b) indicates a relation between the current (I) and the efficiency (dP/dI).
As shown in FIG. 26, according to the semiconductor laser 170, the efficiency (dP/dI) significantly varies with respect to a variation in current, and thereby, stabilized light emission cannot be obtained. As mentioned in xe2x80x9cAnalysis of the mode internal coupling in InGaAs/GaAs laser diodesxe2x80x9d, Laser Physics, Vol. 4, No. 3, pp. 485-492, 1994 written by P. G. Eliseev and A. E. Drakin, the cause is considered to be a resonance of light between the p-side electrode 112 and the n-side electrode 113.
Furthermore, according to the results of researches conducted by the present inventors, it was discovered that the laser light resonated between the p-side electrode 112 and the n-side electrode 113 since the laser light spread in the GaAs substrate 101 was not absorbed by the GaAs substrate 101. Accordingly, the inventors discovered that stabilized laser light emission could be obtained by restraining the spreading of laser light into the GaAs substrate 101 for a reduction in the optical output efficiency (dP/dI), completing the present invention.
Accordingly, the present invention has the object of providing a semiconductor laser that has a great laser light aspect ratio, a high kink level of transition from light emission in a fundamental mode to light emission in a first order mode and a small variation in optical output efficiency (dP/dI).
It is a further object to provide a semiconductor laser in which an increase in threshold current and a reduction in the optical output efficiency (dP/dI) at an elevated temperature are prevented.
The present invention provides a semiconductor laser that includes a semiconductor substrate, an active layer formed on the semiconductor substrate, guide layers laminated on both sides of the active layer, and cladding layers laminated on both sides of the guide layers, wherein a low refractive index layer having a refractive index lower than that of the cladding layer is interposed between the guide layer and the cladding layer, and a total layer thickness of the active layer and the guide layers is not less than about 15 percent of an oscillation wavelength of the semiconductor laser.
By thus inserting the low refractive index layer, the distribution of the emission light can be confined in the low refractive index layer, the guide layer and the active layer. With this arrangement, the light intensity of a near field pattern is increased and the divergent angle of the laser light can be reduced.
This arrangement can also prevent the light emission mode from shifting from the fundamental mode to a higher-order mode, allowing stabilized light emission to be obtained.
This can also prevent the increase in the threshold current and the reduction in the optical output efficiency at an elevated temperature.
A total layer thickness of the active layer and the guide layer should preferably be about 18% of the oscillation wavelength of the semiconductor laser.
By thus setting the layer thicknesses of the active layer and the guide layer, the emission light can be sufficiently confined in the layers. With this arrangement, the increase in the threshold current and the reduction in the light emission efficiency at an elevated temperature can be prevented.
The low refractive index layer should preferably be provided on one side or both sides of the active layer. The above arrangement is adopted because the effect of confining the emission light can be obtained by inserting the low refractive index layer on one side or both sides of the active layer.
The present invention further provides a semiconductor laser that includes a semiconductor substrate, an active layer formed on the semiconductor substrate, a first guide layer laminated on one side of the active layer, a first cladding layer that is laminated on the first guide layer and is at least partially provided with a current constriction portion, a second guide layer laminated on the other surface of the active layer, and a second cladding layer that is laminated on the second guide layer and has a refractive index higher than that of the first cladding layer, wherein a distribution of emission light of the semiconductor laser is shifted so that the emission light has a maximum intensity inside the active layer.
By making the refractive index of the second cladding layer higher than the refractive index of the first cladding layer, the distribution of the emission light is shifted to the second cladding side, and this allows stabilized laser light of a small aspect ratio to be obtained.
By positioning the peak position of the laser light in the active layer, the light emission efficiency can be increased.
The present invention further provides a semiconductor laser in which the refractive index of the first guide layer is made greater than the refractive index of the second guide layer to shift the distribution of the emission light.
By employing the above structure, the peak position of the emission light can be shifted into the active layer.
The present invention further provides a semiconductor laser in which the layer thickness of the first guide layer is made greater than the layer thickness of the second guide layer to shift the distribution of the emission light.
By employing the above structure, the peak position of the emission light can be shifted into the active layer.
The present invention further provides a semiconductor laser in which the band gap of the first guide layer is made greater than the band gap of the second guide layer.
In the above structure, the overflow of electrons from the active layer injected into the active layer can be reduced to allow the light emission efficiency to be increased.
The present invention further provides a semiconductor laser in which the second cladding layer is arranged between the substrate and the active layer and the layer thickness of the second cladding layer is greater than the layer thickness of the first cladding layer.
In the above structure, the distribution of light into the substrate is reduced, and this allows a resonance phenomenon occurring between electrodes to be restrained. Consequently, a variation in the light emission efficiency (dP/dI) is reduced, and this allows a stabilized optical output to be obtained.
The present invention provides a semiconductor laser that includes a GaAs substrate, an InxGa1xe2x88x92xAs (0 less than xxe2x89xa60.3) active layer formed on the GaAs substrate, a first guide layer laminated on one surface of the active layer, a first cladding layer that is laminated on the first guide layer and is at least partially provided with a current constriction portion, and a second guide layer and a second cladding layer that are laminated in order on the other surface of the active layer, wherein a normalized frequency of the first guide layer is made higher than a normalized frequency of the second guide layer.
In the above semiconductor laser, the light intensity distribution into a GaAs layer such as a GaAs substrate or a GaAs contact layer can be reduced, and this allows the resonance of light between the p-side electrode and the n-side electrode to be reduced. With this arrangement, the kink level attributed to a variation in a longitudinal mode with regard to a current-to-optical-output characteristic can be improved, and accordingly, the light intensity variation efficiency (dP/dI) can be restrained. Therefore, a semiconductor laser having a stabilized optical output can be obtained.
The normalized frequency V is defined by the following equation 1:
V=koxc2x7((n12xe2x88x92n22))xc2x7T
where ko: the wave number in a free space (2xcfx80/xcex),
n1: the refractive index of the guide layer,
n2: the refractive index of the cladding layer, and
T: layer thickness of the guide layer.
The first cladding layer may be provided on a side opposite from the GaAs substrate with interposition of the InxGa1xe2x88x92xAs active layer.
By employing the above structure, the present invention can be applied to a ridge type semiconductor laser.
The first cladding layer may be provided on the same side as the GaAs substrate with respect to the InxGa1xe2x88x92xAs active layer, and a GaAs contact layer may be laminated on the second cladding layer.
By employing the above structure, the present invention can be applied to a semiconductor laser provided with an embedded type current block layer.
The layer thickness of the first guide layer should preferably be greater than the layer thickness of the second guide layer.
The above arrangement is adopted because the normalized frequency of the first guide layer can be made greater than the normalized frequency of the second guide layer by making the layer thickness of the first guide layer greater than the layer thickness of the second guide layer, according to the aforementioned equation 1.
The refractive index of the first guide layer should preferably be greater than the refractive index of the second guide layer.
The above arrangement is adopted because the normalized frequency of the first guide layer can be made greater than the normalized frequency of the second guide layer by making the refractive index of the first guide layer greater than the refractive index of the second guide layer, according to the aforementioned equation 1.
The refractive index of the second cladding layer should preferably be greater than the refractive index of the first cladding layer.
By employing the above structure, the aspect ratio of the laser light is improved, and this allows the kink caused by a modal change in the lateral mode of light intensity to be increased with regard to the current-to-optical-output characteristics. With this arrangement, a stabilized light intensity can be obtained.
The layer thickness of the second cladding layer should preferably be greater than the layer thickness of the first cladding layer.
By employing the above structure, a distance between the InGaAs active layer and the GaAs substrate is increased, and-this allows the light intensity distribution into the GaAs substrate to be further restrained.